In West German Offenlegungsschrift 22 32 067, there is described an aqueous acidic phosphatizing solution having a weight ratio of Zn: PO.sub.4 of 1:12 to 110 or, expressed as Zn: P.sub.2 O.sub.5 of 0.11 to 0.012:1, for the treatment of meatl surfaces, particularly iron and steel. The lower zinc content of this bath, as compared to conventional phosphatizing baths, results in improved, thin and uniform phosphate coatings which exhibit good adhesion and stability and which are particularly suitable as a base for the application of a subsequent electrophoretic coating.
Additionally, a process for phosphatizing metals which are then subsequently electroplated is described in West German Offenlegungsschrift 30 04 927. In this process, an aqueous acidic solution is used which contains from about 0.5 to 1.5 g/l of zinc and from about 5 to 30 g/l of phosphate and further contains a nitrite and/or an aromatic nitro-compound. The surfaces to be treated are first immersed in this phosphatizing solution and are then subsequently sprayed with it.
Similarly, in West German Offenlegungsschrift 25 38 347, there is described an acidic aqueous phosphatizing solution which contains at least 0.5% of phosphate ions and at least 0.03% of zinc. In this solution, the molecular weight ratio of phosphate ions to nitrate ions is about 1:0.7 to 1.3 and the molecular weight ratio of zinc ions to phosphate ions is less than about 0.116:1 or, expressed as a weight ratio of zinc ions:P.sub.2 O.sub.5 of less than about 0.107:1.
It has been found that during the operation of the above-described processes, high quality phosphate coatings are formed during the initial stages of operation of the process. As the operation of the process is continued, however, with an increase in the total metal throughput, the phosphatizing results begin to fluctuate and become nonuniform. In some instances, for example, although the phosphate coatings retain their uniform appearance, there is an appreciable loss in the quality of the phosphate coating, particularly with regard to its anti-corrosion protection when used in combination with a subsequently applied paint or similarly organic coating. In other instances, deterioration in the appearance of the coating is also observed. Instead of uniformly gray coatings, irridescent passivating coatings and/or slimy deposits are formed.
It is, therefore, an object of the present invention to provide an improved process that overcomes these deficiencies of the known prior art processes.
A further object of the present invention is to provide an improved phosphatizing process which consistently produces a uniform phosphate coating, having very good corrosion resistance, even with large total metal throughput in the phosphatizing bath.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.